Rheem Furnace cycling, but not heating.

[Cander1]

Neighbors furnace, 1999 Rheem gas pack and based on my non professional opinion, looks to be in pretty good condition for an older unit.
Neighbor is a senior with limited income and from what I understand she called the HVAC repairman that had replaced the heat exchanger last season for help.
He came out, told her she needed to replace the entire system, took her money and left without repairing the unit. I do hope there is a special place in Hell for people that take advantage of seniors.
I see the cost of a furnace repair in my future so she doesn't freeze this winter, which will have to be my course of action this week as the temperatures are cooling down.
I have a background in electronics so troubleshooting electrical equipment isn't foreign, but I am not an HVAC technician.
Fortunately the owners manual has an excellent troubleshooting guide and schematics.
The -
Furnace goes through the start up sequence, starts running then the Limit Control switch in the air plenum opens and the furnace shuts down.
Once the Limit Control switch cools down and closes the furnace will go through the cycle again.
Plenum seems to get real hot, yet the squirrel cage blower is running. I temporarily by passed the LC switch to see if it could be anything else and the unit kept running, so the issue is either the switch or an air flow issue.
I have ordered a new Limit Control switch, but in the back of my mind can't help but wonder if the blower is actually moving enough air. Especially after seeing the specs on the LC switch at 120 degree open and 110 close.
She said she put in a new filter, which I am going to look at to make sure and insure all the vents are open.
For this issue, you have the Limit Control Switch, motor start capacitor, and the blower motor. Of course everything controlled by the electronic control board.
Going to check voltage to the motor to see if it is at the correct level.
How do I know or can I tell if the blower motor is running at the required speed to keep the LC switch from *
I know the blower will spin faster in AC mode but I guess I need to see if I can speed the motor up in Heating mode to see if the LC switch stays closed.
What could cause the blower motor to run at a slower speed, enough to cause the plenum to over *
I have a Trance furnace in my house and keep the blower running most of the time. You can obviously hear it if you go into the crawl space.
But the blower on the neighbors system, seems quiet in comparison. Just can't get the possibility of an air flow issue out of my mind.
Any help, thoughts, or suggestions are welcome.

[Cander1]

Thanks for the reply!
The part number on the existing LC switch is 47-23610-03, which crosses to 470015.
The site where I ordered the new one showed it as "SPST N/C Limit, 120 Open, 100 Close". I thought it low as well, but not a HVAC guy.
This was what got me to thinking it may be a air flow issue because the plenum where the blower sits gets really hot before the switch opens.
She said she put in a new filter, but I am going to verify it and check the grill. Also going to check vents; people think closing off rooms will save heating and cooling cost.
It is likely the evaporator coil has never been cleaned. Looking at it the other day it didn't seem too bad but worth a closer look.
Just thinking that if she didn't change the filter regularly and she likely purchased the cheap filters, the coil could be plugged up enough to cause this.
She won't let me work on it today, (Sunday) so will have to look at it on my lunch hour tomorrow. The new LC switch is due in and I can install it then as well.
I can check the voltage to the motor and hopefully eliminate that.
Thanks again for your help and I'll post what I find.

[Cander1]

Got the new LC switch in today and installed it.
The unit cycled a couple of times at first right after the burners lit, but on the 3rd try stayed lit.
Unit actually ran longer before the LC tripped. May be due to it being cooler outside as well.
The way the unit is configured, with the side cover off, the blower is open to the outside, so this eliminates anything plugged on the inside reducing air flow.
So I suspect the fan isn't pushing enough air.
Checked voltage at L1 and L2 on the control board. 124.5 and 123.9.
Checked voltages at each wire going to the motor. The wires plugged in from top to bottom and marked as follows.
Low - 3.6vac - Red or Orange wire that led back to a terminal on the board labeled Unused.
Hi - 23vac - Black wire that led back to a terminal marked ACC on the board.
2 - 25vac - Orange wire that led back to the start cap < may be the run cap....not sure.
1 - 25.5vac - Brown wire that led back to the other side of the start cap then had a second wire that led back to the board and forgot to get that info.
On the start or run -
Feed from the control board - 123.8
Feed to motor at the Cap - 280vac
Now - the wire that goes from the cap (280vac) I measured at the motor(25.5vac) and it is a direct wire. No transformer.
How the heck can I have 280 at one end of a wire and 25.5 at the * Checked the wire and no resistance. I cannot explain this.
But also can't explain 123.8 on one side of a cap and 280 on the other. A cap passes AC, it doesn't step up voltage like a transformer.

[ryguy85]

I'm not an hvac guy either but I do know electronics as well. That does seem very odd that the voltage would change across a wire. My question is how are you measuring the * If you are trying to compare the voltage at the cap to the motor terminal (i.e. connected by a wire) then you should be using the same reference terminal or ground for each measurement or else you could be measuring a completely different voltage.

[surenuff]

If you have a temperature limit control opening, and your blower is running, then you are probably wasting time checking voltages. It sounds like you are on the right track with low airflow. The unit tag in the furnace should give temperature rise information on the tag. In other words, it will tell you that (ie) 40*-60* is exceptable temperature rise. If you are rising temperature more than these values on the tag, then you probably have low air flow. One way to check to see if you are in the ball park - Most manufactures engineered their equipment to deliver in the neighborhood of 400 cubic feet per minute of airflow on high speed or (cooling speed). Turn your thermostat fan selector from "auto" to "on", then turn on the furnace and let it run for a about ten minutes to stabilize. Then measure air temperature going into the furnace, and check air temperature at the closest spot to the furnace that you can. If you have to, drill a small hole for your temperature probe above the evaporator. If you have to, just check air coming out of closest vent. Formula for air flow will -

Cubic Feet Per * BTU output divided by (1.08 X Temperature Difference)

Your furnace should be an 80% furnace which means that if the data plate in the furnace says 100,000 btu input, then the btu output would be 80%
This should help you determine if your temperature rise is too high, and if so, if airflow is the culprit. ...............................